What Causes El Nino? A Warm Patch of Ocean That Reshapes the World's Weather
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Peruvian fishermen named it centuries ago: a warm current that appeared off their coast around Christmas some years, driving the anchovies away, and they called it El Nino, the Christ child. It turned out to be the local face of the largest year-to-year swing in the planet's climate, a shift in the tropical Pacific that moves rain from Indonesia to Peru, dries out Australia, weakens the Indian monsoon and nudges global temperature up by a few tenths of a degree, all from a change in the wind.
The normal state of the Pacific
In an ordinary year the trade winds blow steadily from east to west across the tropical Pacific, pushing the warm surface water towards Indonesia and piling it up there, so that the sea surface at the western end is around half a metre higher and eight degrees warmer than at the eastern end. Off South America the water dragged away is replaced by cold water welling up from the deep, rich in nutrients, which feeds one of the world's great fisheries.
The warm pool in the west heats the air above it, which rises, forms towering storm clouds and dumps rain on Indonesia, Papua New Guinea and northern Australia. Having risen, the air flows back east high in the atmosphere and sinks over the cool eastern Pacific, where it suppresses cloud and keeps the coast of Peru one of the driest places on Earth. This loop of wind, warm water, rising air and sinking air is called the Walker circulation, and it is what breaks.
When the winds falter
El Nino begins when the trade winds weaken. Nobody has found a single trigger; bursts of westerly wind in the western Pacific and the natural sloshing of the warm pool both seem to play a part. Once the winds slacken, the warm water that was held in the west spreads back eastward across the ocean like water in a tilted bath returning to level, and the cold upwelling off South America shuts down. The eastern Pacific warms by two or three degrees, sometimes more.
The change feeds itself. Warmer water in the east warms the air above it, which weakens the temperature contrast that drove the trade winds in the first place, so the winds weaken further and more warm water moves east. The rising air and the rain follow the warm water into the central and eastern Pacific, and the whole Walker circulation shifts. An El Nino typically builds through the northern summer, peaks in December and fades by the following spring, and it comes round every two to seven years.
What it does around the world
Moving the Pacific's main rain engine thousands of kilometres east rearranges weather far beyond the Pacific, because the tropical storms drive waves in the atmosphere that travel to the poles. The effects are not identical every time, but the pattern is well established:
- •Indonesia, Australia, the Philippines: drought and wildfire as the rain moves away
- •Peru and Ecuador: floods and landslides as the rain arrives, and the anchovy fishery collapses
- •India: a weaker monsoon and a higher risk of failed harvests
- •Southern United States and California: a wetter winter
- •Southern Africa and the Amazon: drier than usual
- •Atlantic hurricanes: fewer, as high-level winds tear developing storms apart
- •The whole planet: a few tenths of a degree warmer, since heat stored in the ocean is released to the air
La Nina, the other half
The oscillation swings both ways. La Nina is the opposite state, with stronger than usual trade winds, a colder eastern Pacific and an intensified version of the normal pattern: heavier rain in Australia and Indonesia, drought in the southern United States and East Africa, more Atlantic hurricanes, and a slightly cooler global average. Scientists refer to the whole swing as ENSO, the El Nino-Southern Oscillation, the second half of the name coming from a seesaw in air pressure between Tahiti and Darwin that Gilbert Walker noticed in the 1920s while trying to predict the Indian monsoon, decades before anyone connected it to the ocean.
The two extremes are not symmetrical. El Ninos tend to be sharper and shorter, La Ninas longer and more likely to repeat for two or three winters running, as happened from 2020 to 2023.
Forecasting and the future
El Nino is the most predictable thing in climate on a timescale of months. A line of buoys moored across the tropical Pacific since the 1990s measures temperatures down to 500 metres, satellites watch the sea surface and its height, and models can now see a developing event six to nine months ahead, which gives farmers, water managers and fisheries time to prepare. The very strong El Ninos of 1982, 1997 and 2015 each pushed global temperature to a record, and the 2023 event did the same.
How the oscillation will behave in a warmer world is one of the unsettled questions in climate science. Models disagree on whether El Ninos will become more frequent, but most agree that the rainfall swings they cause will become larger, so the same event will bring heavier floods and deeper droughts than it did in the past.
The takeaway
El Nino happens when the trade winds across the tropical Pacific weaken, letting warm water that is normally held near Indonesia spread east and taking the region's rain with it, which shifts weather patterns around the globe and warms the planet for a year. La Nina is its colder opposite, the two together form ENSO, and an event can be forecast more than half a year ahead.